Silver Streaks and Black Dots? It's Thrips

Watch the episode · 4:18

Silver streaks on your leaves, with tiny black dots? That's not a deficiency. It's an insect: thrips. They're slim and about 1.5 mm long, and they matter. In a Canadian cannabis grow, just five of them on a young plant meant about a third less harvest. Here is how to spot them, why sprays so often fail, and the hiding place most growers miss.

The short answer ▶ 0:29

Look for silver streaks with black dots. Hang sticky cards: thrips fly, so the cards catch them. Remember that most of them drop into the soil for part of their life. And never lean on one spray. Now, why.

Spotting thrips ▶ 0:44

Adults are slim and yellow to brown, about 1.5 mm long, with fringed wings. The young are paler and have no wings. On cannabis, the usual culprits are western flower thrips and onion thrips.

Source · UConn Extension; Utah State Univ. Extension

They pierce leaf cells and suck out what's inside. The emptied cells leave silvery patches and streaks, dotted with tiny black specks: their droppings. Spider mites leave fine pale pinpricks instead.

Source · UConn Extension

Check the tender new growth, where the young ones hide. Then tap a shoot over a sheet of white paper: thrips show up as tiny moving slivers.

Source · UConn Extension

Sticky cards (and which colour) ▶ 1:26

Unlike spider mites, adult thrips fly, so sticky cards do catch them. Count them every week and you'll see if numbers are climbing.

Source · UConn Extension

In one orchard study, blue cards caught 7 times more western flower thrips than yellow ones. In another study, the type of glue changed which colour won. So either colour works, but blue often catches more.

Source · Broughton & Harrison, Crop Prot. (2012); van Tol et al., Sci. Rep. (2021)

The hidden life cycle ▶ 1:47

Why are thrips so hard to get rid of? Because of their life cycle. Females push their eggs inside the leaf, out of reach of most sprays. The young feed on tender new growth. Then they drop into the soil to pupate, and come back up as adults. In a warm grow the whole cycle takes 1 to 2 weeks, and one female can lay 150 to 300 eggs.

Source · UConn Extension

That soil step is bigger than you'd think. In a 2004 study on bean plants, 98% of western flower thrips left the plant to pupate in the soil.

Source · Berndt et al., Biol. Control (2004)

So a spray on the leaves misses every thrips that's in the soil at the time. In the same study, predatory mites that live in soil killed up to 80% of the thrips there. Predatory mites deserve a closer look of their own.

Source · Berndt et al., Biol. Control (2004)

Onion thrips have another trick: females can reproduce without males. So one thrips riding in on a new plant can start a whole population.

Source · McCune et al., Phytoprotection (2021)

What five thrips did to a cannabis harvest ▶ 2:41

A trial in a Canadian cannabis facility tested how much that matters. Young clones started with zero, one or five onion thrips, and grew sealed in fine mesh bags for 8 weeks.

Plants that started with no thrips gave about 130 g of fresh harvest each. Plants that started with five gave about 90 g: roughly a third less. It was a small, early study, with one variety in one facility. But a few thrips is not nothing.

Source · McCune et al., Phytoprotection (2021)

Why sprays wear out ▶ 3:08

Sprays wear out, too. In greenhouses in south-east Spain, using the same insecticide more than 10 times per crop left thrips highly resistant to it. Extension advice is to rotate between products that work in different ways, about every 2 to 3 weeks. And check what's legal and safe for your plants' stage.

Source · Bielza et al., Pest Manag. Sci. (2007); UConn Extension

What to do this week ▶ 3:26

Tap new growth over white paper once or twice a week. Hang sticky cards and count them weekly. Check every new plant or clone before it joins your grow. And deal with the soil stage too, not just the leaves.

Silver streaks with black dots mean thrips. Sticky cards catch them, most drop into the soil to pupate, and just five of them can cost you a third of your harvest. If you're checking leaves anyway, it's worth knowing how to catch spider mites early too.